With the increasingly improved science and technology, the DC-DC converter is developed toward higher efficiency, higher power density, higher reliability, and higher output current and lower cost. Since a lower voltage power metallic oxide semiconductor field effect transistor (MOSFET) has a smaller on-resistance, the pressure drop is small when a current goes through the MOSFET. Therefore, the MOSFET could function as a rectifying device by replacing a diode, which greatly improves the efficiency of the DC-DC converter. When the power MOSFET is used as the rectifying device, a phase of a voltage between a gate electrode and a source electrode thereof must be kept consistent with a phase of the rectified voltage so as to realize the rectifying function, which is referred to as synchronous rectifying. The synchronous rectifying is widely applied in the DC-DC converter with a low voltage and a large current by virtue of its low conduction loss.
The synchronous rectifying may be divided into an out-driving synchronous rectifying and a self-driving synchronous rectifying depending on the driving modes. The out-driving synchronous rectifying means that driving signals of a rectifier directly comes from an external circuit, for example, a Pulse Width Modulation (PWM) control circuit. The out-driving synchronous rectifying could realize a better waveform and time sequence of the synchronous driving signal. However, the PWM control circuit is provided at the primary side, and outputs the synchronous driving signals to the secondary side for driving the rectifier on the output side. In this case, an isolation device is needed, which results in the increase of the amount of the components and the cost. The self-driving synchronous rectifying means that the voltage outputted from a transformer or an output inductor is appropriately processed and then is used to drive the rectifier. The self-driving synchronous rectifying is widely used because it needs fewer components, has a simple arrangement and a low cost, etc.
As shown in FIG. 1, the conventional self-driving synchronous rectifying apparatus includes a primary circuit having a transfer switch, a PWM control circuit, a transformer having a primary winding Wp and a secondary winding Ws, a rectifying circuit having at least one a rectifier, a filtering circuit, and a self-driving circuit. The primary circuit is electrically coupled to the primary winding Wp of the transformer and the PWM control circuit, and the rectifying circuit is electrically coupled to the secondary winding Ws of the transformer, the self-driving circuit, and the filtering circuit. Under the influence of the parasitic parameters, when the transfer switch is turned on, one rectifier in the rectifying circuit is still in on-state. At this time, another rectifier (may be a synchronous rectifier or a diode) begins to be turned on, and then a shoot-through problem will occur between the two rectifiers, thereby an instantaneous short circuit is occurred in the secondary winding Ws of the transformer, which leads to additional power loss and unnecessary electromagnetic interference, and impacts efficiency of the DC-DC converter.